Dorthy asked: What did Gabriel mean when he told Daniel in Daniel 9:26 that the end of it shall be with a flood, when elsewhere we are told that the end shall be by fire?
The verse you mentioned comes in a tricky section of Scripture, so I want to back up and provide some context for you.
This is a passage where Daniel predicts a prophetic period of "70 weeks." The word "week" in Hebrew means "period of seven." It could refer to seven days or seven years. Both of those could be described with the word "week." Here, Daniel is referring to years. So the time frame of Daniel's prophecy is 70 periods of 7 years, or 490 years.
If you look back one verse, in Daniel 9:25, you'll notice that the clock starts with a command to rebuild the temple. That happened, and we can read about that in the books of Nehemiah and Ezra. Israel rebuild Jerusalem and the temple during that time. Daniel said that period is seven "weeks" or 49 years.
Following that came a second period that Daniel described at the end of verse 25. He said it will be a time of trouble. He indicated that it was a time of waiting for the "prince" or the Messiah. Daniel told us that this second period lasted 62 "weeks" or 434 years. This second period spanned from the completion of the temple to Jesus' arrived in Jerusalem in what we call the Triumphal entry.
It's at this point in Daniel's prophesy where there was a pause or a gap. Israel did not accept Jesus as their Messiah. Jesus was "cut off" as Daniel predicted in Daniel 9:26. These first two periods in Daniel's prophecy are already fulfilled.
If you were following the math so far, you realize there is a missing week. The first period was 7 weeks, and the second period was 62 weeks. That's only 69 and Daniel predicted 70.
Right now, we are living in that gap between the first 69 "weeks" of Daniel's prophecy, and the remaining 1 "week." That final week is coming in the future and we call it the Great Tribulation. So, there is a gap in the prophecy from the Triumphal Entry to the start of the Great Tribulation.
The final week of Daniel's prophecy, the Great Tribulation, is what Daniel describes as chapter 9 closes. He refers to the desolations and abominations that the Antichrist will cause on the earth during that time.
Now, you didn't ask about any of this, but it's important context. Let me get to your specific question. The phrase in Daniel 9:26 that you wondered about says this:
And the people of the prince who is to come shall destroy the city and the sanctuary. Its end shall come with a flood... (Daniel 9:26)
This is a prediction of the destruction of the temple, but Daniel is not referring to a flood of water. You'll notice that the destruction comes at the hand of "the people of the prince to come." The prince to come refers to the Antichrist, and the people of that prince refer to all who reject Jesus. Daniel is referring to a "flood" of people, enemies of God, who destroyed the temple. He's not referring to a flood of water.